Global controls on phosphatization of fossils during the Toarcian Oceanic Anoxic Event

Konservat-Lagerstätten—deposits with exceptionally preserved fossils—vary in abundance across geographic and stratigraphic space due to paleoenvironmental heterogeneity. While oceanic anoxic events (OAEs) may have promoted preservation of marine lagerstätten, the environmental controls on their taphonomy remain unclear. Here, we provide new data on the mineralization of fossils in three Lower Jurassic Lagerstätten—Strawberry Bank (UK), Ya Ha Tinda (Canada), and Posidonia Shale (Germany) —and test the hypothesis that they were preserved under similar conditions. Biostratigraphy indicates that all three Lagerstätten were deposited during the Toarcian OAE (TOAE), and scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) show that each deposit contains a variety of taxa preserved as phosphatized skeletons and tissues. Thus, despite their geographic and paleoenvironmental differences, all of these Lagerstätten were deposited in settings conducive to phosphatization, indicating that the TOAE fostered exceptional preservation in marine settings around the world. Phosphatization may have been fueled by phosphate delivery from climatically-driven sea level change and continental weathering, with anoxic basins acting as phosphorus traps.

with a Nikon d3200 digital single lens reflex camera variably equipped with zoom, wide-angle, and macros lenses. The brightness and contrast of photographs were digitally adjusted using Adobe Photoshop. A combination of microscopic and photographic methods were employed to document the presence of specific tissues in the fossils, including the eyes, guts, and caudal fins of the fishes; the exoskeletal layers of the crustaceans; and the ink sacs, gladii, and mantle tissues of the coleoids.

Scanning electron microscopy and energy-dispersive X-ray spectroscopy
Scanning electron microscopy (SEM) and energy dispersive X-Ray spectroscopy (EDS) were used to characterize the microstructures and elemental compositions of the fossils 1-3 , including the authigenic and diagenetic minerals that they contain due to preservational processes 4 , like phosphatization, pyritization, calcification, and aluminosilicification 5 . The electron microscopy work included secondary electron SEM (SE-SEM), which produces with high topographic contrast, and backscattered electron SEM (BSE-SEM), which produces images known for compositional contrast based on the average atomic number (Z) of each material in a sample 1,3 . The EDS analyses were conducted to collect semi-quantitative data on the concentrations of elements in the various fossil materials; these data are presented in the form of elemental maps.
Small specimens from Strawberry Bank (3 cm or smaller across its longest axis) were analyzed at the Electron Microbeam Laboratory at the Department of Geological Sciences, University of Texas at Austin (UT Austin), using a JEOL-6490LV SEM equipped with a tungsten filament electron source, BSE and SE detectors, and an EDAX Apollo 11 silicon-drift EDS detector. The small specimens from the Posidonia Shale were studied at the Materials Analysis, Testing, and Fabrication (MATFab) Facility at the University of Iowa using a Hitachi S-3400N SEM equipped with a tungsten filament electron source, BSE and SE detectors, and XFlash silicon drift detector. All large specimens (i.e., specimens larger than 3 cm along their longest axis) were analyzed at the X-ray Microanalysis Core Laboratory, University of Missouri (Mizzou), using a Zeiss Sigma 500VP SEM equipped with a Schottky field emission electron source, a 5-segment high-definition solid-state BSE detector (HDBSD), a cascade current low vacuum secondary electron detector (C2D), an Atlas 5 correlative microscopy workflow system, and dual Bruker XFlash 6|30 silicon-drift EDS detectors. This SEM system was used to acquire large 'mosaic' images through an automated process of gathering and combining numerous smaller images, thereby allowing for the study of large specimens (and large surface areas), which cannot be visualized using common and conventional SEM instruments.
All samples analyzed were uncoated and unpolished, with the exception of one Uncina posidoniae lobster claw (Specimen number NPL00036039.000), which was discovered in a polished slab that was donated to researchers at the State Museum of Natural History Stuttgart.
At all facilities, the SEM chamber pressure was held at low vacuum (1-35 Pa) during the analyses, allowing chamber gases to disperse electron charge; this dispersal eliminated the need for the damaging application of conductive coatings 1,3 . Given the use of three SEM instruments, operating conditions inherently varied, though efforts were taken to ensure comparable analytical parameters. At UT Austin and MATFab, BSE images were acquired with an accelerating voltage of 20 keV, spot size of 45-50, and a working distance between 9-24 mm. At Mizzou, all SEM work was conducted with a beam accelerating voltage of 20 keV, beam current of 40 nA, a 60 μm aperture, and a working distance of 16-20 mm. Large image mosaics of both BSE and SE signals, using the HDBSD and C2D respectively, were assembled from full fossil surfaces using the Atlas 5 workflow.
Similarly, SEM-EDS was conducted to obtain elemental concentration variation at UT Austin, MATFab, and Mizzou with differences in operating conditions. At UT Austin, spectral and areal elemental mapping data collected using the EDAX Genesis Apollo 11 system were processed using the EDAX Genesis spectrum software (Version 6.43), with operating conditions as follows: beam accelerating voltage 20 keV, working distance 10-12 nm, and spot size 99.
These settings yielded X-ray count rates of over 100,000 counts per second. Likewise, at MATFab, the elemental data collected by the XFlash silicon drift detector were analyzed using Quantax software and the following operating parameters: beam accelerating voltage 20 keV, working distance 10-12 nm, and spot size 99. Under these conditions, the X-ray count rates were between 10,000 and 30,000, which are sufficient for elemental analysis. At Mizzou, the operating conditions used for SEM imaging were maintained for SEM-EDS analyses, with the exception of the aperture, which was increased to 120 μm to ensure higher X-ray count rates.
Spectral and areal elemental mapping data were collected using the dual, coplanar Bruker XFlash 6|30 EDS detector units in tandem, yielding X-ray count rates on the order of 300,000 counts per second. Elements that were mapped included aluminum (Al), barium (Ba), carbon (C), calcium